Phase-inverter circuit



Feb. 1, 1944. R. Smm@ 2,340,611

PHASE y INVERTER CIRCUIT Filed June 28. 1941 @WF1/'r Gmb :mentor Robert 'anford Gttorneg Puendelirl, 1944.

PHASE-INVERTER CIRCUIT `Robert C. Sanford, Camden, N. J., assigner to Radio Corporation of of Delaware America, a corporation Application June 28, 1941, Serial No. 400,268 6 Claims. (Cl. 179-171) verter tube, `and the ratio of these voltages,

Ea/Eb, as a, measure of the amplitude bal-ance of the phase inverter circuit, is made equal to unity by adjusting the portion of the voltagethat is applied to the grid of the inverter tube. In the usual circuit, this voltage is derived from a tap on the anode coupling resistor of the preceding voltage amplifier tube, through a coupling capacitor.

Variations betwenrtubes of the same type used in the inverter stage and variations in the value of the anode coupling resistors constituting the,

voltage divider network from which the phase inverter voltage is derived, have been found to produce corresponding variations in the ratio Ea/Eb and, accordingly, a self-balancing phaseinverter circuit has in some cases found a preference in use. This circuit provides degenerative feedback by a common resistor without by-pass in the anode circuit of the single-ended or driver stage and in the anode4 and grid circuits of the inverter stage.

The magnitude of the signal voltage across the common coupling resistor, which is applied `to the grid of the inverter tube, depends upon the difference between the values of the output signal currents of both the single-ended or driver stage and the inverter stage. Hence, the effects of variation in the value of the common coupling resistor or the effects of possible variations' between diiferent tubes of the same type used in the inverter stage is relatively-small.

Because of the degeneration resulting from a4 portion of the output of the inverter being fed back to the input of the inverter, the stability characteristic of degenerative amplifiers is obtained without appreciable loss of overall gain through the amplifier, including the balanced or push-pull output stage. The balance in the voltages applied to the output stage is within limits which are acceptable, the ration Ea/Eb being approximately 1.1 in most cases. As the gain of the inverter stage is increased, however, the ratio more nearly approaches unity, I

The present invention relates more particularly' to the self-balancing type of phase inverter circuit, and has for its primary object to provide an improved circuit of that type which is selfregulating and self-balancing while, at the'salne time, permitting the'rato Ea/Eb to more nearly approach unity Iand to maintain that ratio with variations in coupling impedance values and in variations between various tubes of the same type in the inverter stage. l

It is also an object of this invention, to provide an improved self-balancing phase inverter circuit including a. resistor network in place of the usual common coupling resistor, whereby a relatively small portion of the output signal at the driver or singlefended amplifier stage is made to appear at the inverter grid to cancel the voltage applied thereto, and whereby the unbalance in the amplifier may be substantially zero.,

The invention will be understood further from the following description, when considered in connection with the accompanying drawing, and its scope is pointed out in the appended claims.

In the drawing, Y'

Figure 1 is a. schematic circuit diagram of a self-balancing phase-inverter network embodying the invention, and y Figure 2 is .a similar schematic circuit diagram showinga portion of the circuit of Fig. 1, modified in accordance with the invention.

Referring to Fig. 1, 5 is a driver or input amplifier stage for the inverter network connected with a single-ended input circuit t for audio frequency signals, for example, and coupled through a suitable impedance coupling network 'I to the input grid 8 of one of a pair of balanced amplifier tubes S and Ill comprising the output' or power amplifier stage. The input grid of the amplifier I Il is indicated at II. Both grids are provided with suitable resistors I2 and input coupling capacitors I3 in connection with' the anode circuits It and I5 of the driver 5 and inverter' stage I6, respectively.

In the usual inverter circuit, the input grid Il of the inverter stage I6 is coupled through ya voltage divider network including a resistor I8,

to the anode circuit I4 of the driver stage or the grid circuit of the output stage, selected so that the gain from the coupling resistor I8 to grid I I is unity. With known circuits of the self-balancing type, a permanent slight unbalance is produced.

In the present case. the inverter grid is connected through a lead I9 to a terminal 28 which is connected tothe low potential ends oi both grid resistors I2|2 and to a common ground through a resistor 2| connected between the terminal 20 and the chassis or ground return 22 for both the grid and anode circuits of the amplifier. Blas potential is applied to the grids 8,A and from a suitable self-bias resistor-23 connected lbetween ground and cathode leads 24 of amplifiers 9 and I0. This resistor is provided with a suitable audio frequency bypass capacitor 26.

The value of the resistor 2| may be substantiallyy twice the normal value for a self-balancing -sistor 30 which, in effect, tends to decouple the terminal 29 from the terminal 20 and the anode phase-inverter amplifier system, since it is sub`-l stantially in parallel with a similar resistor 28 connected Ibetween a terminal 29 and ground 22,

through a series coupling or degeneration cons,v

trolling resistor 30 connected -between the terminals' 20 and 29. If the voltage Ea equals the voltage Eb there will be no feedback and zero potential will correspond to the mid-tapon the resistor 30 provided by the two resistors2| and 28 at the ground connection. If an unbalance exists, it will bereduced -by the resulting regeneration or degeneration which occurs.

The present resistor network provides a connection to ground or chassisfor the grid 8 of the output tube 9 through the coupling resistor I2 and the resistor 2|, while a similar connection to ground is provided for the grid II of the output tube I through its corresponding grid resistor and the resistor 28. 2| and 28 may preferably be of the same order,

for example, 250,000 ohms, and likewise the resistors I2 may be of substantially the same value.A

The decoupling or feedback control resistor 30 may be of a suitable value forI controlling the feedback and, in the present example, may be considered to be of the order of 250,000 ohms.

The grid of the phase inverter tube I6 is supplied with signal voltage through the resistor network `I2-2I-3Il---28-I2. It is desirable to design the circuit so that Ea as nearly as possible is equal to Eb and 180 out of phase. In order to do this the Voltage on the grid II must be g times smaller than Ea where g is the gain of the inverter stage.

The self-balancing phase-inverter network comprises the resistor 28 together with the resistors 30 and 2|. Since the signal voltage from the inverter stage I6 is developed across the network comprising the resistor 28 in parallel with the resistors 30 and 2| in series. as its load resistance, the voltage fed back lby the' network to the inverter grid Il is less than the voltage across the resistor 28 by an amount determined by the ratio of the sum of the resistances at 30 and 2| with respect to the resistance at 2 I.

Accordingly, there is less cancellation of the voltage on the inverter grid than normal to inverter circuits of the self-balancing type, and, for any given value of gain of the inverter, the resistances 2|, 28 and 30 may assume values suchthat the voltages at the output grids 8 and II are substantially equal and 180 out of phase under all conditions of operation.

The resistor network 2|, 28 and 30 is connected between ground or chassis and the terminal 20, which is common Ato the anode circuit of the driver tu-be and to the anode and grid circuits of the inverter tube I6. connection, the magnitude of the signal voltage across this network which is applied to the grid of tube I6 depends on the difference between the values of the output signal currents of the tubes ,circuity of the' inverter tube from the grid 'circuit thereof .to apredetermined degree.

The radio Ea`/Eb for the grid voltages applied to the output'tubes 9 and I0 is made substantially equal to unity with this self-balancing cir- Thus, the two resistors cuit, by adjustment of the network resistances at output transformer 35, and the bias potentialv applied to the control grid Il of the inverter stage may be derived from any suitable source. In the present example, the bias potential on the grid I1 is controlled by an adjustable tap connection 38 on a voltage supply resistor 31 having -a negative terminal connected to chassis or ground, as indicated at 38, with the control means connected to the cathode 39. A bypass capacitor`4|l is provided for the cathode connection. In this manner, the grid I1 is placed at a suitable negative potential with respect to the ing a filter capacitor 4'I connected to chassis or ground at the output end thereof, and the control grid |1 of the inverter stage I6 is coupled to the terminal 20 through a blocking or coupling capacitor 48. Bias potential is-applied to the grid Il through a grid resistor 49 connected to ground, from a self-bias resistor 50 in circuit with the cathode 39. A bypass capacitor 5I Because of this common is provided for the resistor 50, as inthe preceding circuit arrangement.

In this modification of the circuit, the cathode of the inverter stage is returned to a volt'- age source such as the self-bias resistor 50, whereby the net voltagel difference between the grid and the cathode of the inverter is correct for the condition of operation desired. In this -case .both the output grids and the inverter grid may be operated independently o f each other. at

the proper potentials for most efficient and distortionless amplification.

I claimvas my invention:

1. In a phase-inverter amplifier of the selfbalancng type, the combination of a pair of balanced output amplifier tubes each having an input grid circuit and a grid resistor connected in said circuit, a feedback coupling resistor connected between i said grid resistors, means providing a high resistance connection to ground for each of said grid resistors, and means providing a phase inverter grid connection with the ground end of one of said grid resistors;

2. A self-balancing phase-inverter system comprising a driver amplifier stage, an output amplifier stage including an amplifier tube impedance-coupled to the said driver stage, an inverter amplifier stage including an amplifier tube having a cathode, a control grid and an anode, a second output amplifier tube in said output stage impedance-coupled to said inverter stage, a grid resistor for each of said output amplifier tubes, means for coupling the output of said last named amplier tubes in balanced relation to each other, a coupling resistor connected between each of said grid resistors and ground, a feedback control resistor connected between the grid ends of said last coupling resistors, and means providing a coupling connection between one end of said feedback resistor and the control grid of said inverter stage amplifier tube.

3. In a phase-inverter amplifier of the selfbalancing type, the combination of a driver amplifierstage, a balanced output amplifier stage, an inverter stage having a signal input grid and an output anode, means providing aresistance coupling network between said driver stage and one side of the balanced output stage and between the inverter stage and the other side of the balanced output stage including an anode coupling connection for the inverter stage, means providing a connection for the inverter stage grid with said network to provide substantially unity gain between the driver stage output and the input of said other side of the balanced output stage, and means providing a resistive conl nection in said network for decoupling the anode and grid of said inverter stage to a predetermined degree thereby to establish and maintain a predetermined condition of balance in `said amplifier.

4, A phase-inverter amplier of the self-balancing type comprising in combination a pair of ampliner'tubes having a balanced input circuit, means providing a resistance path across each half of said input circuit, a driver stagecoupled to one-half of said input circuit, an inverter stage having a signal input grid and an output anode coupled to the other half of said input circuit, a feedback control resistor connected between predetermined spaced terminals Y on said resistance paths, and a' grid connection for said inverter stage with one of said terminals.

5,. In a' phase-inverter amplifier, the combination of an audio frequency ampliiier tube, an output amplier tube coupled to said rst-named tube, a phase-inverter tube, a second output amplier tube coupled to said phase-inverter tube in pushpull relation to the first-named output tube, a resistor network comprising two relaancing vcoupling network comprising input grid connections for a balanced amplifier, a bias potential supply connection for said ampliiier, impedance coupling means providing a conducting path between said last-named connection and said input grid connections in balanced relation, an impedance element providing a bridging connection between two points on said paths, an inverter -amplier stage coupled to said iirstnamed network to apply signals thereto, and means providing a control grid connection for said inverter stage with said network to derive" a signal voltage for'said stage through rsaid impedance element.

' ROBERT C. SANFORD. 

